From long ago, the separation of gas using a gas separation membrane has attracted attention as being a simple and facilitative technique capable of separating a mixture gas continuously while maintaining a gaseous state, the technique not involving a phase change. The separation of gas is a technique for selectively separating a gas with the aid of the presence or absence of permeation and the differences in permeation rate according to the kind of gas (hereinafter, sometimes referred to as gas) which is to permeate through a gas separation membrane.
As materials for the gas separation membrane, there have been known some polymers such as cellulose acetate, polysulfone, polyimide and the like. Of these, polyimide is known as a material having a strength suitable for being used for a gas separation membrane, resistant to breakage, and excellent in heat resistance so as to be usable at high temperatures.
Gas separation membranes using polyimide have variously been reported, in which the influence of the structure of a monomer on gas separation performances such as the permeability of the membrane for separating a target gas, the high selectivity of the target gas and the like is eagerly studied.
For example, it is known that a polyimide-based gas separation membrane containing a hexafluoroisopropylidene group (—C(CF3)2—) in a repeating structure have high permeability of helium (hereinafter, sometimes referred to as He) and carbon dioxide (hereinafter, sometimes described as CO2) and therefore the selectivity of these gases against oxygen (hereinafter, sometimes described as O2) and methane (hereinafter, sometimes referred to as CH4) is highly exhibited.
Additionally, in producing a gas separation membrane by introducing a hexafluoroisopropylidene group (—C(CF3)2—) into a repeating unit of polyimide, it is said possible to weaken the intermolecular interaction while enhancing the rigidity of the chains to cause a difference in permeability of a gas separation membrane according to the kind of gas thereby accomplishing both a high membrane permeability and a high selectivity (see Non-Patent Publications 1 and 2).
However, materials for synthesizing a hexafluoroisopropylidene group-containing polyimide include only the following diamine and carboxylic dianhydride. These are limited in chemical structure when developed into a polyimide membrane, so that it is difficult to design a chemical structure with consideration paid to the strength and separation performance as observed in use as a gas separation membrane. Furthermore, there is a problem of limitation of organic solvents in which polyimide is soluble. Incidentally, the separation performance of a polyimide membrane containing a hexafluoroisopropylidene group, with regard to a mixture gas of He, CO2, O2 and CH4 is discussed in Non-Patent Publication 3.

In Patent Publications 1 to 3, a fluorine-containing polymerizable monomer for polymerizing a fluorine-containing polyimide, which is a diamine having 2-hydroxy-1,1,1,3,3,3-fluoroisopropyl group (—C(CF3)2OH) (hereinafter, sometimes referred to as a HFIP group) is disclosed together with a method for producing the same.
As a method for producing a gas separation membrane formed containing polyimide and the like, there are known a method of obtaining a uniform membrane by applying a polyimide solution in a wet condition and then only by evaporating a solvent and a method of obtaining a nonuniform asymmetric membrane formed having a dense layer and a porous layer. The method of obtaining a nonuniform asymmetric membrane is exemplified by a method of discharging a polymer solution through an outlet and evaporating a solvent disposed in the vicinity of a surface in air to form a dense layer, in which method the surface is thereafter immersed in a coagulation bath charged with a coagulation liquid (which is a solvent compatible with the solvent for the polymer solution but not compatible with the polymer) thereby forming a finely porous layer in the coagulation bath. In Patent Publication 4, a method for producing a multilayer reverse osmosis membrane according to the above method is disclosed.
As discussed above, diamine and carboxylic dianhydride for polymerizing a hexafluoroisopropylidene group-containing polyimide are limited in chemical structure when developed into a polyimide membrane, as discussed above, so that it is difficult to design a chemical structure with consideration paid to the formability, strength and separation performance of a gas separation membrane.